An electronic amplifier is a device for increasing the power of a signal. In basic operation, an electronic amplifier uses energy from a power supply and controls its output to match an input signal shape but with a larger amplitude. Amplifiers are used in many electronics applications, including in communications, audio systems, servo motor controllers, and many other applications.
A key operational parameter of any amplifier is its gain. A gain is typically a multiplicative factor relating the magnitude of the output signal to the input signal. The gain may be specified as the ratio of output voltage to input voltage (voltage gain), output power to input power (power gain), or some combination of current, voltage and power. In some applications, it is beneficial that an amplifier have a variable gain, wherein the gain of the amplifier may be modified or changed. However, existing approaches to adapting conventional amplifiers to provide for variable gain have disadvantages.
To illustrate, reference is made to FIG. 1, which depicts a conventional amplifier 100, as is known in the art. A conventional amplifier may comprise transistors 108a, 108b, drain resistors 112a, 112b, and current source 106. Each drain resistor 112a, 112b may be coupled between the drain terminal of a corresponding transistor 108a, 108b, respectively, and a high potential voltage source (e.g., VDD). Each transistor 108a, 108b may in turn be coupled at its gate terminal to a differential input (e.g., Vin+ or Vin−) and at its source terminal to current source 106.
In operation, transistors 108a, 108b form a differential transistor pair to sense and track the input data represented by a differential input voltage Vin, and producing a differential output voltage Vout at the drain terminals of transistors 108a, 108b. Those of skill in the art will appreciate that the gain A of amplifier 100 may be given by the equation A=gm1·RD, where gm1 is the transconductance of each transistor 108a, 108b and RD is the resistance of each resistor 112a, 112b. Thus, to make the gain A variable, one or both of gm1 and RD must be made variable. For example, RD may be variable if resistors 112a, 112b are replaced with variable resistors. As another example, those of skill in the art will appreciate that gm1 is a function of ISS, the tail current generated by current source 106. Accordingly, gm1 may be varied by varying ISS, which it turn varies the gain A. However, as those of skill in the art will appreciate, the common mode output voltage Vcm, out of amplifier 100 may be given by the equation Vcm, out=VDD−ISS·RD/2. Thus, adjusting the gain by changing either of gm1 or RD will lead to a change in the output common mode voltage. Changing the output common mode of amplifier 100 may be undesirable, as it disturbs the bias condition of the circuit.